Nasal masks currently are employed for various purposes, including for the delivery of oxygen to persons who suffer lung disease or who are exposed to rarefied atmospheres, for administering anaesthesic gases and for delivering pressurised air to persons who suffer from such disorders as sleep apnea. The masks usually are moulded from a relatively soft, resilient plastics material and they are shaped during manufacture to match the facial contours of an average intended wearer. However, a problem with the known types of masks is that, because individuals vary so much from the average, the masks must be forced against their inherent resiliency to deform and so adapt to the shapes of the users in order to avoid gas leakage. This requires that the masks be secured firmly by retaining straps or harnesses in order to prevent air leakage and, depending on the degree of deformation required in any given case, may produce discomfort, irritation or even ulceration of the upper lip and/or the nasal bridge where there is little cushioning from subcutaneous tissue. Thus, the retaining force normally is distributed over a relatively small sealing area defined by the peripheral edge of the mask and this causes a relatively high localised pressure to be exerted on the face of the wearer. Also, because the sealing area is relatively small in the currently employed masks, minor dislocation of a mask may produce a significant leakage path for gas.
Attempts have been made to overcome the above difficulties by shaping perimeter edges of some masks in such a manner that the edges tend to move with a rolling action when a mask is pushed into contact with a wearer's face. This shaping permits differential movement around the perimeter of a mask and facilitates adaptation of the mask to the facial contours of the wearer.
Also, U.S. Pat. No. 4,971,051 discloses a mask which has been developed in an attempt to provide both comfortable and conformable perimeter sealing, the mask having a flexible pneumatic cushion formed around its perimeter and/or being provided with a so-called flap ring which is formed as an adjunct to the mask. The flap ring comprises a flexible membrane which is affixed to the perimeter of the mask and it acts in the manner of a flap valve when blown against a wearer's face by air delivered to the interior of the mask.
A problem which is inherent in a mask of the type disclosed in U.S. Pat. No. 4,971,051 is that the flap ring is constrained by its attachment to the perimeter seal of the mask and its freedom to adapt to facial contours is limited. Moreover, again because the flap ring is attached to the perimeter seal of the mask, any movement of the perimeter seal may cause movement of the flap ring relative to the wearer's face and thereby open a path through which gas may leak. Similarly, movement of the flap ring may cause stresses to be imposed on the perimeter seal, so that a path may be created through which gas may leak.